Flexibility is most quickly developed by building strength in extended positions. For this, you need full range-of-movement strength exercises (hamstring and adductor pulldowns, flies, etc) and isometric stretches (also called PNF or CRAC). When you sufficiently develop flexibility in your kicking antagonists (the muscles which resist during the movement), you should focus on building strength in the agonists (the muscles which contract and lift the leg). In a reverse crescent kick, these include the hip flexors and abdominals. Simply practicing the kick slowly (1/2 to 1/4 normal speed) will do for starters.

lift your knee , make sure u dont start kicking until your hip is about 15 degrees off target then your hip will allow your leg to go in the right direction and height. you should post a vid of u doing the kick, then we could see your mistake within it.